1. Field of the Invention
The present invention relates to an exhaust gas purification device for an engine, and more particularly to an engine exhaust gas purification device comprising a filter for trapping particulate matter contained in the exhaust gas, and an EGR (exhaust gas recirculation) valve for recirculating the exhaust gas to an intake side.
2. Description of the Related Art
Restrictions on the exhaust gas of diesel engines become stricter year by year, and hence the discharge amount of particulate matter (PM) contained in the exhaust gas must be suppressed. A continuous regeneration diesel particulate filter (DPF) exists as a device for removing PM from exhaust gas.
As shown in FIG. 2, in a continuous regeneration DPF, a filter c for trapping the PM in the exhaust gas is provided in an exhaust passage b of an engine a. The PM trapped in the filter c is burned continuously according to the temperature of the exhaust gas, and thus the filter c self-regenerates. However, when the exhaust gas temperature is low, for example under low speed, low load conditions, the PM trapped in the filter c cannot be burned by the temperature of the exhaust gas, and hence regeneration is not possible. As a result, the PM continues to accumulate in the filter c, causing the filter c to become clogged and the exhaust pressure to rise.
To solve this problem, a technique of using a catalyst-carrying filter c′ as the filter c such that the unburned components of the fuel are supplied to this catalyst-carrying filter c′ is known. According to this technique, the catalyst-carrying filter c′ is activated to rise in temperature by the unburned fuel components, and hence the catalyst-carrying filter c′ can be regenerated forcibly even when the exhaust gas temperature is comparatively low, for example under low speed, low load conditions.
Supply of the unburned fuel components to the catalyst-carrying filter c′ has been achieved by the present inventor and so on through multi-injection and post-injection from an injector d into the cylinder. Multi-injection involves performing one or more sub-injections following a main injection while the flame generated by the main injection continues to burn. Post-injection involves performing one or more sub-injections following the main injection after the flame generated by the main injection has died out.
If an EGR valve e is opened to implement exhaust gas recirculation during forcible regeneration of the catalyst-carrying filter c′ through such multi-injection and post-injection, the unburned fuel components produced by the multi-injection and post-injection are recirculated to the intake side from the exhaust side through an EGR passage f. As a result, the unburned components turn into a tar-like substance and stick to an intake manifold g and the like. In the worst case, this may lead to blockages. Hence, a system in which the EGR valve e is closed and the exhaust gas is not recirculated during forcible regeneration of the catalyst-carrying filter c′ has been considered.
However, it was discovered that with such a system, although unburned components can indeed be prevented from adhering to and accumulating in the intake manifold g by closing the EGR valve e, exhaust gas remains in the EGR passage f, an EGR cooler h, and so on, which are disposed further upstream in the flow direction of the EGR gas than the closed EGR valve e, and hence the unburned components turn into a tar-like substance and accumulate in these parts h, f.
Note that Japanese Patent Application Laid-open Publication S58-51235 (Patent Document 1) and Japanese Patent Application Laid-open Publication H3-67014 (Patent Document 2) are known as related prior art documents. However, in the device disclosed in Patent Document 1, the filter c is self-regenerated by controlling an intake throttle valve i provided in an intake passage h to close, and in the device disclosed in Patent Document 2, in addition to the control described in Patent Document 1, self-regeneration of the filter c is improved in efficiency by opening the EGR valve e to reduce the amount of new intake air, thereby increasing the temperature of the exhaust gas passing through the filter c. Hence the technological premise of these documents differs from that of the system described above, in which self-regeneration is performed by raising the temperature of the catalyst-carrying filter c′ through multi-injection and post-injection.